1. Field of the Invention
This invention relates to a torque compensation circuit for d.c. motor drives utilizing motor field weakening for speed control.
2. Description of the Prior Art
In industrial applications wherein two speed regulated drives are connected together by means of the material being processed, such as a metallic strip, metallic wire, synthetic fibre tow or the like, tension in the material must be controlled to prevent excessive sag or stretch beyond the elastic limit of the material. In multi-stand mills or strip reduction mills, speed regulated stands are used with interstand tension control apparatus to insure constant tension. In some installations it is not feasible or economically justifiable to employ elaborate interstand tension control, and a current droop is added to the speed regulated drives. Thus the speed vs. current characteristic for each motor has a drop such that under equilibrium conditions, i.e. constant speed for each motor, they run at different armature currents. Since armature current is a good indication of motor torque for motors with fixed excitation, a current droop implies torque droop and provides an acceptable interstand tension control.
However, it has been observed that the motor current signal does not represent torque for motors which have extended speed range by field weakening, and accordingly, torque must be calculated from motor current and field flux intelligence at each operating point.